During an Annual Inspection a Mechanic with an IA (Inspection Authority) rating reports discovering severe exfoliating corrosion on both sides of a left landing gear bulkhead stiffener forging on a Cessna 150B. A major repair was required involving removing the left landing gear and replacing the entire bulkhead stiffener forging. Allegedly; there was no evidence of moisture; fuel; oil or hydraulic fluid contamination.
Synopsis
During an Annual Inspection a Mechanic with an IA (Inspection Authority) rating reports discovering severe exfoliating corrosion on both sides of a left landing gear bulkhead stiffener forging on a Cessna 150B. A major repair was required involving removing the left landing gear and replacing the entire bulkhead stiffener forging. Allegedly; there was no evidence of moisture; fuel; oil or hydraulic fluid contamination.
Narrative
During an Annual Inspection on a Cessna 150B aircraft I discovered what appeared to be delamination on the landing gear bulkhead stiffener assembly. The landing gear bulkhead assembly is made of solid aluminum so I know it could not be delamination; but a result of corrosion. On initial inspection I could see that the lower row of rivets on the outboard side (the bucktails) were completely eroded away from the corrosion that had caused the aluminum to separate. Upon removal of the landing gear I could see much more damage that was present. On the lower portion of the component; the aluminum bar that supported the weight of the landing gear while the aircraft is airborne (the aluminum) had swelled up and separated from deep corrosion that had the appearance of delamination of a composite structure. Large pieces of aluminum had been flaking off for some time and had eroded between 1/16 to 1/8 of an inch in depth. With further inspection of the component; it was found that corrosion was present in the upper portion of the aluminum and had also began to swell; which in turn caused one rivet head to be sheared off from the tension that was put on the rivet. Parts of the upper portion of the [stiffener] component that was beginning to swell had also developed cracks from the expansion of the aluminum.
NASA callback
Reporter stated the corrosion was on the left main landing gear bulkhead stiffener. The aircraft had been built in 1962. He was doing the second Annual Inspection since the owner had bought the aircraft. A different IA Inspector had performed the previous inspection for the same owner. Reporter stated the bulkhead stiffener is a half inch thick of solid forged aluminum. The Service Manual; Section 19-52; states that no cracking or damage is allowed. There are no pictures in the Service Manual. Mechanics have to use the Cessna Illustrated Parts Catalog (IPC) Figure 11B; Item #3; to get a breakdown of the components. Mechanics have to use the 1963-1969 Service Manual because the aircraft Serial Number is included in that grouping. Reporter stated access to the lower area of the stiffener is a little difficult because a mechanic has to look up through the opening in the lower fuselage where the spring gear (main gear) comes through; after removing a collar and gasket that covers the opening on the lower external fuselage around the spring gear. That area of corrosion on the lower section of the stiffener cannot be seen from inside the aircraft. But the upper section of the same bulkhead stiffener and spring gear can be seen from under the pilot seats after removing access panels; corrosion had already started on the upper end. Reporter stated he found one-eighth of an inch deep corrosion on each side of the stiffener. But an 'L' bracket that was attached to the stiffener was not corroded at all; even though the stiffener was swelling and flaking-off aluminum at the same location. The spring gear comes through a forward to aft elongated hole in the bulkhead stiffener with wedges that secure the spring gear in the stiffener; allowing landing loads to be transferred to the upper section of the stiffener. Reporter stated the entire area of the left bulkhead stiffener was dry; no evidence of water or moisture accumulation; no fuel; oil; or hydraulic fluid or lines were in that area. All three landing gear wheels had wheel pants (fairings) installed; which drastically reduced the amount of water that would normally splash up on the gears. But he still had to remove the entire floorboard area inside the Cessna 150B to perform a major repair involving removing the left main gear and replacing the entire bulkhead stiffener forging.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.